Or unless fundings and lack of customers
But ok for real the F-35A is only as equivalent as a stealthy F-16 in terms of maneuverability, so if the J-21 possesses good air characteristics that's at least equivalent of a flanker, then only thing standing between both planes would be range, payload, stealth, speed, and BVR capabilities(radar). If 611(J-21 if I recall correctly) is intended to compete against the 601 J-20, and the J-20 is speculated to be in the same class as F-22, then J-21 should at least be as capable as Eurofighter, Su-30MKK. If logic applies, then J-21 would definitely outperform F-35 in WVR at the very least. As for stealth while F-35 probably comes around 3rd or 4th (after B-1, F-22, J-20?) in terms of frontal RCS, this also remains a question whether PLA J-21 would have an equivalent of frontal RCS, since it's meant for competing against the 601. Speed-wise I'm inclined to believe the J-21 will be more than Mach 1.6(F-35), and weapons will probably be around 6 (F-35A is 4 internal, which means it is small and short-legged in terms of range, speed, and payload). However of course it's my guesses, so we will see how far things go.
Anyways given the day is still early to pinpoint the layout of Liaoning's air fleet, there's nothing for certain. However if rumors are true that another 5th gen is coming out, it might be for PLAN, then we will really begin to see how far all of this will fall in place. What we are more sure of would be F-35's specs, so it will be a good basis for comparison. And of course depending which breed of F-35 we are talking about, I won't be surprised we will draw very different results.